1. Field of the Invention
The present invention relates to a noise reduction processing apparatus and a noise reduction processing method for reducing noise in a moving image composed of a plurality of frames, and more particularly to a noise reduction processing apparatus and a noise reduction processing method involving three-dimensional noise reduction processing in which noise is detected across a plurality of temporally consecutive frames. The present invention also relates to an image sensing apparatus provided with such a noise reduction processing apparatus.
2. Description of Related Art
In recent years, with the development of various digital technologies, image sensing apparatuses, such as digital cameras and digital video recorders, that shoot digital images with a solid-state image sensing device such as a CCD (charge-coupled device) or CMOS (complimentary metal oxide semiconductor) sensor and display apparatuses, such as liquid crystal displays and plasma televisions, that display digital images have been becoming increasingly widespread. To reduce noise in a moving image composed of a plurality of frames in such image sensing apparatuses and display apparatuses, various noise reduction technologies have been proposed.
One of such noise reduction technologies is two-dimensional noise reduction processing, in which noise reduction is performed within a single-frame image. In two-dimensional noise reduction processing, a single-frame image is processed with a spatial low-pass filter so that the pixel values of a pixel of interest and of surrounding pixels are smoothed and thereby noise in the target frame is reduced. Since this two-dimensional noise reduction processing is noise reduction processing within a single frame, it has the disadvantage of susceptibility to the pixel values of surrounding pixels, resulting in a blurred image. Instead, the signals of two temporally consecutive frames may be added up so that those frames are processed with a temporal low-pass filter thereby to reduce noise. This is three-dimensional noise reduction processing, another known noise reduction technology.
The configuration of a conventional three-dimensional noise reduction processing apparatus that performs such three-dimensional noise reduction processing is shown in FIG. 22. As shown in FIG. 22, the conventional three-dimensional noise reduction processing apparatus comprises: a noise detecting portion 31 that detects noise by comparing the image signal of the current frame acquired by a solid-state image sensing device with the image signal of the preceding frame; a motion detecting portion 32 that detects a motion amount by comparing the image signals of two frames, namely the current and preceding frames; a feedback coefficient calculation portion 33 that sets a feedback coefficient based on the motion amount detected by the motion detecting portion 32; a multiplication portion 34 that multiples the noise acquired by the noise detecting portion 31 by the feedback coefficient from the feedback coefficient calculation portion 33; a subtraction portion 35 that subtracts the noise acquired by the multiplication portion 34 from the image signal of the current frame acquired by the solid-state image sensing device; and a frame memory 36 that temporarily stores the one-frame-worth image signal that has undergone noise elimination by the subtraction portion 35.
In the conventional three-dimensional noise reduction processing apparatus configured as described above, when the image signal that will form the current frame is inputted, the noise detecting portion 31 calculates the difference between it and the image signal of the preceding frame stored in the frame memory 36, and detects the amount of noise based on the difference. Moreover, the motion amount detecting portion 32 detects the amount of subject motion between the image signals of the current and preceding frames. Then the feedback coefficient calculation portion 33 calculates a feedback coefficient such that the greater the motion amount the smaller the feedback coefficient, and feeds it to the multiplication portion 34.
Thus the multiplication portion 34 multiplies the noise acquired by the noise detecting portion 31 by the feedback coefficient calculated by the feedback coefficient calculation portion 33, thereby calculates the true noise recognized between the current and preceding frames, and feeds it to the subtraction portion 35. Thus the subtraction portion 35 subtracts the noise calculated by the multiplication portion 34 from the image signal of the current frame, and thereby outputs a noise-reduced image signal of the current frame. The image signal of the current frame thus noise-reduced is outputted to the stage succeeding the three-dimensional noise reduction processing apparatus, and is also fed to the frame memory 36 so as to be temporarily stored there for noise reduction of the image signal of the next frame.
Some proposed image sensing apparatuses that perform three-dimensional noise reduction processing as described above perform both still image shooting and moving image shooting and in addition are capable of efficient noise reduction in still image shooting. In these image sensing apparatuses, three-dimensional noise reduction as described above is performed on RGB signals before matrixing for generating luminance and color-difference signals. When the RGB signals having undergone three-dimensional noise reduction processing are then subjected to matrixing, the luminance signal is assigned a greater number of pixels in still image shooting than in moving image shooting to acquire a high-quality, high-definition image.
Disadvantageously, however, in the three-dimensional noise reduction processing shown in FIG. 22 and the three-dimensional noise reduction processing in the image sensing apparatuses described above, an image signal of the same resolution as the image signal of the processing target frame (current frame) is necessary as the image signal of the preceding frame for processing. Specifically, to store the image signal acquired from the solid-state image sensing device in the frame memory, the frame memory needs to have an amount of memory corresponding to the number of pixels of the solid-state image sensing device.
Thus, in the conventional three-dimensional noise reduction processing apparatus, using a solid-state image sensing device of high resolution with a view to acquiring an image signal that will form a high-resolution image requires that the image signal of the preceding frame stored in the frame memory needs to be of high resolution with a large amount of data. This not only increases the memory capacity of the frame memory for storing the image signal of the preceding frame but also increases the amount of data transferred between the frame memory and different blocks in the three-dimensional noise reduction processing apparatus. This increases the size of the apparatus, and in addition increases the power consumption in three-dimensional noise reduction processing.